Much time and effort has been devoted to the study of solar radiation and to methods of capturing this inexhaustible source of solar energy to provide power for terrestrial and space applications. One technique for capturing and using this energy is to arrange a plurality of reflective panels, such as highly reflective aluminum or silver facets, into a large parabolic array to form a solar array concentrator. Large parabolic arrays of this type, several meters in diameter, are capable of achieving concentration ratios as high as 2000 to 1. The solar energy is focused on a heat receiver positioned in front of the array. This receiver transfers the energy to a dynamic system including a turbine and an alternator to generate electrical energy. Alternatively, it transfers the energy to a propulsion source such as cryogenic hydrogen to control the movement and the positioning of a satellite, shuttle or space station in orbit.
Higher solar flux concentration levels are obtainable by using a secondary concentrator upstage from the receiver. The secondary concentrator typically is in the shape of a compound curvature parabola in the form of a truncated cone having a ratio of inlet to outlet diameters between 2:1 to 4:1, thereby permitting increases in the flux concentration to 4000:1 or 8000:1.
The successful operation of the secondary concentrator is dependent on the use of such a parabola having a highly reflective surface, often in an environment where temperatures in the secondary concentrator can reach 500.degree. C. or more, at a vacuum of 10.sup.-5 to 10.sup.-7 torr.
Heretofore, highly reflective surfaces have not been capable of withstanding these conditions. Silver has generally been recognized for its ability to provide a highly reflective surface. However, it tends to form oxide films at elevated temperatures thereby decreasing its specular reflectivity. Furthermore, a silver layer having a thickness greater than a few thousand angstroms tends to undergo crystal growth, resulting in an increase in diffuse reflectivity and a decrease in specular reflectivity. This results in a gradual undesirable scattering of the reflected solar rays.
Silver reflective coatings applied over polished copper substrates provide high specular reflectivity at lower temperatures. However, at the elevated temperatures encountered in solar flux concentrators, there is a tendency for the silver to migrate into the copper, thereby causing the reflective surface to become diffuse and ineffective.
Highly specular reflective surfaces can be produced by applying a silver layer over a single crystal sapphire substrate, and the reflectivity remains constant even at elevated temperatures encountered in a secondary concentrator. However, it is difficult to form a single crystal sapphire into the compound curvature of a parabola, and thus the composite has limited use.
U.S. Pat. No. 4,235,951 describes a reflector for solar energy, said reflector comprising a glass layer having, on one side, a support attached with an adhesive, and on the other side, a reflective layer. Silver and aluminum are noted as the preferred reflective material, at a thickness less than 1 micron (10,000 .ANG.).
U.S. Pat. No. 4,547,432 discloses a method of making mirrors by attaching a silver layer to a glass surface using a chemical covalent bond between the silver and the glass. A protective layer of polymethylmethacrylate may be applied over the silver.
U.S. Pat. No. 5,019,458 describes a mirror for solar energy concentrators having high weather and abrasion resistance. The mirror comprises a glass substrate, a NiCr layer about 100 .ANG. thick, a silver layer having a thickness of 700 to 1000 .ANG., a 100 .ANG. layer of ZnS, an alumina layer of about 1 micron thickness and finally, a layer of silica having a thickness of 1000-1500 .ANG.